A recent study discovered that bats jam each other's sonar by emitting siren-like short calls; this interferes with the echolocation of their competition. This is the first time a species has been observed jamming other members of its own species (except, I suppose, for the case of navy engineers jamming enemy's sonar). In the past, however, others have found that certain prey species, such as moths, jam the sonar of predators. The "jamming" basically acts as an acoustic blindfold, confusing or blocking the signals captured by echolocation.

I have found that the best written description of echolocation I've ever seen comes from Calvin and Hobbes (although Calvin's understanding of bats is far from perfect!)

{

Mexican free-tailed bats make short waaoowaaoo sounds that sabotage each other’s sonar-guided aim in duels over the right to gulp a flying moth out of the night sky.
Tadarida brasiliensis, like other aerial hunting bats, locates its prey by making little calls and listening for any echoes bouncing off a moth. Aaron Corcoran of Wake Forest University in Winston-Salem, N.C., was recording other bat sounds when he picked up the strange wavering sirenlike calls in sequences that suggested that free-taileds might be jamming each other.